1. Field of the Invention
The present invention relates to an intra-oral camera for use in dentistry according to the preamble of claim 1.
2. Related Technology
In dentistry, optical photographs of the teeth to be examined are often produced for diagnostic reasons, since any required therapeutic measures can be conveyed to the patient more effectively on the basis of an optical image. The so-called intra-oral cameras used for this purpose are in this case used both for macro photographs, i.e. for example for photographing cavities of an individual tooth, and for a range of photographs from photographs of individual teeth to a full-face photograph. In the past, the use of a constant focal length was provided both for intra-oral and for extra-oral applications, wherein preferably a very short focal length was used, as this was the best solution for intra-oral applications on account of the naturally limited space conditions. Nevertheless, in cheaper optical systems, such a short focal length causes marked distortion; this led to very limited usability of the system for full-face photographs. Ultimately, a camera of this type allows only a very limited observation range to be sharply imaged.
In order to improve the quality, of the optical representations both in the close and in the remote range, it was also known to carry out an adjustment of the image plane of the photographing unit or the image sensor in accordance with the selected photographing situation. This adjustment could be carried out for example manually via the focusing of the imaging system, for example by displacing a group of lenses along the optical axis. Alternatively thereto, it is also possible to carry out the focusing, i.e. the adjustment of the position of the image onto the sensor plane, in an automated manner, wherein the contrast determination of the image signal can be used as an evaluation variable.
The region to be observed by the camera is lit up conventionally with the aid of an illumination unit which is integrated into the camera or attached to the input of the camera in a suitable manner. In close-up photographs of this type, much more reflected light strikes the image sensor on account of the marked reflections of the teeth in conjunction with the short distance of the object in cameras used intra-orally. For this reason, it is beneficial to vary the diaphragm or aperture of the optical system as a function of the available light in order to increase what is known as the depth of sharpness. In this connection, EP 1 058 860 describes a dental camera in which there is a fixedly predefined coupling between the diaphragm and focus via a functional element. Nevertheless, the drawback of this known fixed coupling of the diaphragm opening to the position of the focus lens is the inability of the system to carry out an optimum adaptation to the prevailing conditions. Thus, a fixedly predefined diaphragm in the macro range often does not lead, on account of the different absorption of light of the objects to be photographed, to optimum exposure of the sensor. The absorption of amalgam is for example several times higher than the absorption of dentine. For sharp representation of depth cavities in dentine, a small diaphragm diameter, for example, is therefore advantageous for a high depth of sharpness. Nevertheless, if this macro setting is used to examine a highly absorbing amalgam filling, the signal detected by the sensor has to be strongly boosted on account of the high absorption of the amalgam, as a result of which the signal-to-noise ratio is impaired. If a larger diaphragm diameter is used, on the other hand, local “overexposure” of the image can occur, so that individual pixels of the image recorder become saturated; ultimately, this prevents the object from being sufficiently recognizable.